因为(m^2+n^2)^2=m^4+2m^2n^2+n^4,
(m^2-n^2)^2=m^4-2m^2n^2+m^4,
(2mn)^2=4m^2n^2,
m^4+2m^2n^2+n^4=(m^4-2m^n^2+m^4)+4m^2n^2
所以为直角三角形